In general, an intake valve or an exhaust valve of an engine of a car and the like is opened and closed by a valve train for transmitting rotation of a cam rotating together with a camshaft and operating the valve. Here, for the purpose of suppressing loss of kinetic energy and occurrence of wear of parts due to friction, in some cases, a roller follower which is a plain bearing or a rolling bearing is arranged in a valve train at a portion coming in contact with an outer circumferential surface of the cam and receiving rotation of the cam. In this roller follower, occurrence of damage at a surface of an outer ring, an inner ring (a shaft) and rolling elements constituting the roller follower gives rise to a problem.
In order to address this, such measures as control of a state of stress or hardness at the surface of the outer ring, the inner ring (the shaft) and the rolling elements, surface treatment such as nitriding of the surface, and the like have been proposed. Thus, durability of the surface of the outer ring, the inner ring (the shaft) and the rolling elements constituting the roller follower could be improved to thereby improve durability of the roller follower (for example, see Japanese Patent Laying-Open No. 2000-230544 (Patent Document 1), Japanese Utility-Model Laying-Open No. 5-32702 (Patent Document 2), and Japanese Patent Laying-Open No. 2003-307223 (Patent Document 3)).    Patent Document 1: Japanese Patent Laying-Open No. 2000-230544    Patent Document 2: Japanese Utility-Model Laying-Open No. 5-32702    Patent Document 3: Japanese Patent Laying-Open No. 2003-307223